Recent developments in pipeline construction include the use of automatic or at least semi-automatic welding apparatus and equipment at plural stations, to make girth welds in a series of separate steps for each joint. Such welding is usually accomplished by traveling electric arc welding devices which orbit around or part way around each joint. These heat the pipe end metal and fuse electrode material into the joint step by step, in such a manner as to cause the weld metal to fuse intimately into the pipe end metal. The preferred systems accomplish the welding in a series of sequential steps, using several welding stations at sequential pipe lengths along the line. In a particularly preferred system, a new length of pipe is first secured to the front end of the line by an internal "stringer bead" forming operation. In this, one or more internally orbiting devices inside the pipeline form a partial weld at the inner surface of the pipe which is strong enough to hold the parts together until reinforcing or supplemental weldments can be made. This internal bead also serves as a backing to prevent subsequent weld passes from projecting or burning through to the interior, which would result in weld flaws.
After the internal weldment has been thus produced, as described more particularly in Nelson and Sims, U.S. Pat. No. 3,461,264, for example, by the internal apparatus, which also comprises a clamp for holding the adjacent pipe ends in proper welding position during stringer bead formation, the clamping means are relaxed and the welding apparatus is moved to the front end of the line to weld the next pipe length in place. Electric generating means must step forward from joint to joint in similar manner. The consecutive pipe lengths are usually around 40 feet (about 10 meters) long but, in some cases, "double jointing" is practiced, wherein two lengths of 40 foot pipe are secured together before installation in the line, making the field-installed welds 80 feet or about 20 meters apart. In some cases, even triple-jointing is practiced, to further reduce the number of welds that must be made in the field. In such cases, of course, the consecutive welds made in the field would be about 120 feet or about 30 meters apart.
While the first or internal stringer bead is being welded, supplemental filling and capping weldments are being performed simultaneously at one, two, three or more stations behind the front of the pipeline. Each of these stations requires a power source. In the preferred system, externally orbiting automatic welding instruments, such as the general type of apparatus described in a U.S. Pat. No. 3,806,694 to Nelson, Pollock and Randolph, are being used to perform the supplemental welding operations. Each arc welding instrument is mounted on a tensioned flexible guide track in the form of a strong metal band tightly secured around the pipeline at a location accurately spaced from the joint to perform a precision joint filling or completing operation. Such guide band and traveling carriage means to support the welding instrument and guide it in its orbital travel with high accuracy, are preferably similar to those described in U.S. Pat. No. 3,604,612 to Miller and Nelson. However, in some cases, other welding, carriage and guide means may be used. In any case, it is usually preferred to weld by moving each of two instruments half way around the pipe circumference. One instrument serves to form a weldment half way around for this particular pass, and a separate instrument is moved orbitally around the other half circumference. Therefore, two welder instruments and two separate sources of electric power for each station may be required.
In a typical large diameter pipeline, the girth joints may require several weld passes in addition to the first or "stringer bead" welding pass mentioned above, including a so-called "hot pass". In this pass, the internal part of the joint is highly heated and weld metal is fused, deposited into and firmly bonded to the pipe metal. A so-called "filler pass" may next be inserted, at a station farther back, to further fill the gap between the pipe ends outside of the "hot pass"; this may be followed by a "capping" or joint completion pass. With heavy wall pipe, two or more "filler" passes outside the "hot pass" may be required and, in some cases, a secondary or finishing "capping" pass may be desired. Therefore, in order to complete a welded joint, the internal or first pass may be followed by two, three or more external operations, each of which requires for each instrument a separate power source at the station, with a pair of half circle orbiting welding instruments, each with its own carriage on a guidetrack or band being operated at each station. Thus, except for the front or inner weld forming station where a single generator may suffice, there will be required a pair of generators at each station, the whole job requiring three, four or more generating sets, a pair of generators being required at most of the stations. These generators are needed near their respective stations, whether mounted on the line, or along it, at pipe length intervals. The intervals, as noted above, may be 40 feet, 80 feet, or more.
In the past, it has been common practice to mount a generator, with its individual diesel driver engine or other prime mover, on a heavy track laying tractor or to transport it on its own wheels by means of a tractor along the pipeline from station to station as welding proceeds. Each of these machines including the tractors costs many thousands of dollars, so that very heavy investments are involved. Because of the size and type of tractors, it has been necessary to build a solid roadway along the pipeline to support them. While it has been known, in the past, to mount certain apparatus appurtenant to pipelining on the pipeline itself, such has been limited in the past generally to small, light weight apparatus, such as spray coating devices and the like. An important object of the present invention is to substantially reduce pipeline construction costs, and capital equipment costs, by using much smaller simpler carriage vehicles without requiring tractors and using the pipeline itself as a roadway for the relatively small vehicles. Another object is to insure stability of the equipment on the line by making it self steering and/or self stabilizing. A further object is to provide emergency braking or locking means to secure the carriage to the line. Further objects of this invention will become apparent as this description proceeds.